1. Field of the Invention
The present invention relates to a method of driving a thermal line printer which performs thermal recording with a thermal line head, and to a thermal printer.
2. Description of the Related Art
Thermal line printers are known which perform thermal recording of images, characters, etc., on a heat-sensitive sheet of a predetermined size by using a thermal line head having a plurality of heating resistors arranged in a row. Ordinarily, in printers of this kind, the thermal line head is driven and controlled by being divided into blocks according to a driving method related to a semiconductor integrated circuit (IC) for driving the head.
That is, as shown in the block diagram of FIG. 2, a thermal line head 3 has, for example, 384 (64 dotsxc3x976 blocks) heating resistors R arranged in a lateral row, a shift register 30 which holds serially-input print dot data corresponding to one line, a latch register 20 which holds groups of one-line print dot data items supplied parallel with each other from the shift register 30, a selection circuit 10 formed of NAND circuits for selectively driving the heating resistors R in the printing blocks according to the print dot data from the latch register 20 while being timed on the basis of strobe signals STB1 to STB6 from a central processing unit (CPU) of a drive controller, a thermistor for detecting the temperature of a head portion, etc. Currents are caused to flow through the heating resistors R selected from the 384 heating resistors R according to the printing data to perform desired line-by-line pattern printing on a heat-sensitive sheet. Strobe signals STB1 to STB6 are signals for determining ON/OFF conditions of the heating resistors R.
When the amount of printing data (the number of dots to be printed) is large, considerably large power is consumed for energization of the heating resistors R if all the heating resistors to be driven are simultaneously energized. In such a case, a large and high-cost power supply unit is required. A method has therefore been practiced in which the heating resistors R for printing one line are not simultaneously energized but partially energized with respect to each of a plurality of printing blocks (e.g., six blocks 1 to 6) into which the heating resistors R are separated.
More specifically, drive pulses phase-shifted as shown in FIG. 8, for example, are ordinarily applied for energization. That is, drive pulse P10 is applied block 1, drive pulse P11 to block 2, drive pulse P12 to block 3, drive pulse P13 to block 4, drive pulse P14 to block 5, and drive pulse P15 to block 6, each pulse being phase-shifted from the preceding one by the amount corresponding to the one-pulse width.
After the completion of printing of one line in the above-described manner, the heat-sensitive sheet is fed one step in intermittent feeding, and printing of the next line is subsequently performed, thus performing printing on the entire surface of the heat-sensitive sheet.
However, it has been found that in a case where printing is performed by applying drive pulses P10 to P15 to the resistors R in printing blocks 1 to 6 while shifting the phase of each pulse by the amount corresponding to the pulse width as described above, a problem arises that a white gap line W is generated between the blocks 1 to 6, as shown in FIG. 9. FIG. 9 shows the result of solid-tone printing through one line in such a manner as to emphasize generation of white gap W.
A study made to find the cause of occurrence of such a white gap W has revealed that a nonuniform temperature distribution occurs in each of printing blocks 1 to 6 such that the temperature is higher at a center and is lower at the opposite ends, and the heat-sensitive sheet is not sufficiently heated at each block boundary. FIG. 10 is a graph showing temperature rising conditions of the heating resistors. A curve A represents the rise in temperature of one adjacent pair of the heating resistors at the center of one printing block when the heating resistors are simultaneously driven, and a curve B represents the rise in temperature of the adjacent pair of heating resistors at the adjacent ends of two printing blocks when the heating resistors are driven not simultaneously with each other.
As can be understood from this graph, a temperature difference T is caused between the center A and the end B of the printing blocks. It is thought that this temperature difference T is the cause of occurrence of white gap W at the ends of each adjacent pair of blocks, at which the temperature in the temperature distribution is lower. That is, in the case where thermal line head 3 arranged as shown in FIG. 3 is divided into six blocks 1 to 6 to separately print six groups of dots each consisting of 64 dots in 384 dots for one line, each of the pairs of the heating resistors corresponding to 64th and 65th dots, 128th and 129th dots, 192nd and 193rd dots, 256th and 257th dots, and 320th and 321st dots are not simultaneously driven. Therefore, heat applied to form these dots escapes to the adjacent-dot side, so that the amount of heat applied is insufficient for color development on the heat-sensitive sheet, resulting in occurrence of white gap W.
In view of the above-described problem, an object of the present invention is to provide a thermal line printer drive method which prevents occurrence of a white gap during printing, and a thermal line printer capable of preventing occurrence of the white gap.
To achieve this object, according to one aspect of the present invention, there is provided a method of driving a thermal line printer in which a plurality of heating elements (thermal line head 3, heating resistors R) arranged on a line perpendicular to a sheet feed direction are separated into a plurality of blocks (blocks 1 to 6), and in which the heating elements in each block are driven separately from those in other blocks to perform thermal recording on a heat-sensitive sheet, the method comprising applying a drive pulse (P1 to P6) to each heating element in each block a certain number of times by dividing the drive pulse and applying the divided pulses with a time shift.
This method enables the temperatures of the heating elements to be made generally uniform even though some adjacent pairs of the heating elements are not simultaneously driven, thereby preventing occurrence of the white gap, i.e., failure to develop the color on the heat-sensitive sheet at certain positions. Moreover, applying the drive pulse a certain number of times is effective in increasing the temperature of each heating element to a level high enough to sufficiently develop the color on the heat-sensitive sheet.
Preferably, the method also includes a comparison step of comparing the number of dots to be printed in one line and the largest number of energization dots for one line, and a printing method selecting step of selecting, on the basis of the result of comparison made in the comparison step, one of a division printing method of applying a drive pulse to each heating element in each block a certain number of times by dividing the drive pulse and a batch printing method of simultaneously applying drive pulses to the heating elements in the blocks. The largest number of energization dots is the largest number of heating resistor elements simultaneously energized, which number is selected to set an upper limit of power consumption.
For example, in the printing method selection step, the division printing method is selected when the number of dots to be printed in one line is comparatively large, while the batch printing method for high-speed printing is selected when the number of dots to be printed in one line is comparatively small. Thus, the optimum printing method can be selected according to printing conditions.
The drive pulse in the division printing method may be applied by being divided into drive pulses having a predetermined minimum pulse width determined by a current and a resistance value to make the temperatures of the heating elements generally uniform.
Further, groups of the blocks to which drive pulses are applied during the same time period may be determined according to the number of dots to be printed in one line. The efficiency with which the drive pulses are applied to the heating elements can be improved thereby.
Also, groups of pulses may be successively applied to the blocks in one group with a phase shift between the blocks. This is effective in making the temperatures of the heating elements in one group generally uniform.
According to another aspect of the present invention, there is provided a thermal line printer in which a plurality of heating elements arranged on a line perpendicular to a sheet feed direction are separated into a plurality of blocks, and in which the heating elements in each block are driven separately from those in other blocks under the control of drive control means to perform thermal recording on a heat-sensitive sheet, the printer comprising time division application means for applying a drive pulse to each heating element in each block a certain number of times by dividing the drive pulse.
This arrangement enables the temperatures of the heating elements to be made generally uniform even though some adjacent pairs of the heating elements are not simultaneously driven, thereby preventing occurrence of a white gap, i.e., failure to develop the color on the heat-sensitive sheet at certain positions. Moreover, applying the drive pulse a certain number of times is effective in increasing the temperature of each heating element to a level high enough to sufficiently develop the color on the heat-sensitive sheet.
According to still another aspect of the present invention, there is provided a thermal line printer in which a plurality of heating elements arranged on a line perpendicular to a sheet feed direction are separated into a plurality of blocks, and in which the heating elements in each block are driven separately from those in other blocks under the control of drive control means to perform thermal recording on a heat-sensitive sheet, the printer comprising shift application means for separating the heating elements into a plurality of blocks and for applying drive pulses by phase shifting the pulses with respect to the heating elements in different blocks in one group. This arrangement is effective in making the temperatures of the heating elements generally uniform.
According to a further aspect of the present invention, there is provided a thermal line printer in which a plurality of heating elements arranged on a line perpendicular to a sheet feed direction are separated into a plurality of blocks, and in which the heating elements in each block are driven separately from those in other blocks under the control of drive control means to perform thermal recording on a heat-sensitive sheet, the drive control means including comparison means for comparing the number of dots to be printed in one line and the largest number of energization dots for one line, printing method selecting means for selecting, on the basis of the result of comparison made by the comparison means, one of a division printing method of separating the heating elements into a plurality of groups and for applying drive pulses to the heating elements and a batch printing method of simultaneously applying drive pulses to the heating elements in the blocks, and time division application means for applying each pulse a certain number of times by dividing the pulse in the case where drive pulses are applied by the division printing method. In this arrangement, the division printing method can be selected when the number of dots to be printed in one line is comparatively large, and the batch printing method for high-speed printing can be selected when the number of dots to be printed in one line is comparatively small. Thus, the optimum printing method can be selected according to printing conditions.
The drive control means may also include number-of-blocks determination means for determining the number of blocks into which the heating elements are separated according to the number of dots to be printed in one line. The efficiency with which the drive pulses are applied to the heating elements can be improved thereby.